I've just verified it works, paying with a VISA Debit card. You have to download the GameStop app, the US shop's version of Steam, to install the games. I'm downloading and installing Far Cry 3 now.

It's a US-led offer (the website says it displays the price in pounds converted from dollars) but at the time of publication it is possible to get all three games in the open world shooter series for dirt cheap if you're on these shores.